Recently, there has been a growing demand for video tape of higher image quality. Also, there has been a growing demand for a floppy disk of higher densification as well as higher discrimination. To meet such demands, a magnetic layer comprising ferromagnetic particles is used. In order to increase the characteristics of an audio tape, it is known to use a tape comprising plural magnetic layers superposed upon each other. For example, JP-B-63-56608 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes plural magnetic layers in which an upper magnetic layer comprises metal and a lower magnetic layer comprises iron oxides. Other examples of such tapes having plural magnetic layers are described in JP-B-63-56608, JP-A-63-94431, JP-A-60-256917, JP-A-60-124025, JP-A-54-159203, JP- A-53-16604, JP-A-51-474021, JP-A-58-56231, JP-A-58-53024, JP-A- 58-17539, JP-A-57-143734, JP-A-56-34145, JP-A-54-145104, and JP-A-52-9401 (the term "JP-A" as used herein means an "unexamined and published Japanese patent application").
In order to maintain a good running property which is prerequisite for a magnetic recording medium having good electromagnetic characteristics, it is necessary that the coefficient of friction of the magnetic layer surface be small.
In order to increase the running property of a magnetic recording medium by decreasing the coefficient of friction of the magnetic layer surface, it is known to add lubricating agents to the magnetic layer. For example, in a magnetic recording medium which comprises an upper magnetic layer containing a large quantity of lubricating agents such as fatty acids and a lower magnetic layer being formed on a support and containing no lubricating agents, it has turned out that in the beginning, good running property continues due to the presence of the lubricating agents; however, with the elapse of time the lubricating agents move to the bottom portion of the lower magnetic layer, thereby significantly decreasing the running property of the magnetic recording medium.
Such a transfer of lubricating agents extensively occurs when the magnetic recording medium such as an audio tape or a video tape is allowed to stand or is used under conditions of high temperature. Even if a magnetic recording medium has good electromagnetic characteristics, when the running property thereof is lowered, such electromagnetic characteristics are not fully exhibited.
The present applicant proposed a magnetic recording medium in which the average content of lubricating agents in the upper magnetic layer is smaller than that in the lower magnetic layer. When a difference in the concentration of lubricating agents is provided in magnetic layers, lubricating agents in the lower magnetic layer, the content of which is large, move gradually to the upper magnetic layer in which the content of lubricating agents is small, with the result that the upper surface of the magnetic layers is always supplied with a desired amount of lubricating agents.
Lubricating agents present in the upper surface of the magnetic layers gradually adhere to a running system, so they are rubbed off. Therefore, the average content of lubricating agents in the upper magnetic layer is always smaller than the maximum content of lubricating agents in the lower magnetic layer, and consequently lubricating agents in the lower magnetic layer continue moving toward the upper magnetic layer over a long period of time. As a result, the lubricating property of the magnetic layer surface stays good, so that the magnetic recording medium exhibits an excellent running property. Since the amount of lubricating agents in the upper magnetic layer is not increased to excess, binders in the upper magnetic layer are rarely plasticized by lubricating agents. Therefore, when the magnetic recording medium is in contact with a running system, the surface of the magnetic layer is hardly damaged, so that the running durability as well as electromagnetic characteristics thereof are rarely lowered.
When ferromagnetic alloy particles are used for forming the upper magnetic layer, it is difficult to produce a magnetic recording medium in which sensitivity and S/N are high.
It can be assumed that this is because ferromagnetic alloy particles are liable to aggregate in a magnetic coating composition, and accordingly an aggregated mass is formed while the magnetic coating composition is dispersed, coated, and dried, thereby damaging the smoothness of the magnetic layer surface. It is not clear why the ferromagnetic alloy particles aggregate; however, it is thought that the molecules of the binders being adhered to the surface of magnetic particles are replaced with low molecular compounds having polar groups, thereby decreasing steric hindrance. It is thought that especially, fatty acids or polyfunctional low molecular compounds are apt to accelerate the aggregation of magnetic particles. On the other hand, when fatty acids of strong polarity, which seem to cause aggregation, are not used, the running property of the magnetic tape deteriorates.
Because of the above reasons, it is difficult to obtain a good quality magnetic recording medium such as a video tape, the surface of which is required to be smooth. Also, in a tape comprising plural magnetic layers, the upper magnetic layer usually has a thickness of 2 .mu.m or less; therefore, it is particularly liable to be influenced by the above-mentioned aggregated mass. As a result, it is difficult to increase sensitivity and S/N.